Athletes have long sought ways to improve their performance through a variety of often complex training regimens. Optimizing training and competition by avoiding fatigue and not significantly exceeding anaerobic threshold is extremely important to improve performance.
Physiologic monitoring can be used to assess whether athletes are reaching their fatigue and anaerobic threshold (i.e. exceeding their exercise capacity). The most common monitoring metric is heart rate. When athletes approach or reach a predetermined target heart rate or heart rate range (zone), they can modify their exertion level (e.g. pace) to avoid fatiguing too soon. A variety of portable devices including watches with wireless heart rate sensors are being used to measure heart rate during exercise. However, heart rate is an imperfect metric and has significant variability due to influence from endogenous and exogenous factors.
Another method of measuring or evaluating fatigue in a subject may be temperature. Fatigue may occur at, for example, repeatable, predictable core body and muscle temperature thresholds. However, skin temperature is significantly less effective at predicting fatigue due to influence from the ambient temperature and other factors.
Previous studies evaluating core and muscle temperature during exercise utilized invasive measurements of temperature such as rectal, invasive muscle probes and/or oral methods. These standard methods of measuring core and muscle temperature are cumbersome and most people would not utilize them during exercise. Similarly, while microwave radiometry has been utilized in temperature measurement including measuring temperature from specific locations from a satellite, such microwave radiometry devices are typically large and not portable. For example, radiometers have been disclosed in e.g., U.S. Pat. Nos. 4,346,716, 4,647,281, 4,774,961, 5,334,141, 5,662,110, 5,779,635; and 5,983,124, hereby incorporated by reference.
Currently, there appears to be no system or device that permits non-invasive, comfortable assessment of core body or muscle temperature during exercise. Further, there is an on-going need, especially for athletes, for devices and methods of assessing or monitoring e.g. fatigue during exercise.